Thin-film magnetic recording media with dual intermediate layer structure for increased coercivity

ABSTRACT

Ion beam-deposited, nitrogen-doped C:H films having substantially lower resistivities than undoped ion beam-deposited C:H films and suitable for use as hard, abrasion-resistant overcoat layers for magnetic recording media, such as hard disks, are formed by supplying a mixture of hydrocarbon and nitrogen gases to an ion beam generator. Nitrogen atom content of the films is controlled to within from about 5 to about 25 at. % by appropriate selection of the ratio of hydrocarbon gas flow to nitrogen gas flow. The resultant IBD i-C:HN films exhibit a reduced tendency for charge build-up thereon during hard disk operation by virtue of their lower resistivity vis-à-vis conventional a-C:H materials.

CROSS-REFERENCE TO PROVISIONAL APPLICATION

This application is a divisional of application Ser. No. 09/982,937filed Oct. 22, 2001, now U.S. Pat. No. 6,689,425, which is a divisionalof application Ser. No. 09/400,187 filed Sep. 21, 1999, now U.S. Pat.No. 6,312,798.

This application claims priority from provisional patent applicationSer. No. 60/101,843, filed Sep. 25, 1998, the entire disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a novel, nitrogen-doped,abrasion-resistant material useful as a protective overcoat layer for amagnetic recording medium, e.g., a hard disk, a method of manufacturingsaid material, and a method of controlling the amount of nitrogen dopingof the material.

BACKGROUND OF THE INVENTION

A magnetic recording medium, e.g., a hard disk, typically comprises alaminate of several layers, comprising a non-magnetic substrate, such asof Al—Mg alloy or a glass or glass-ceramic composite material, andformed sequentially on each side thereof, a polycrystalline underlayer,typically of chromium (Cr) or Cr-based alloy, a polycrystalline magneticrecording medium layer, e.g., of a cobalt (Co)-based alloy, a hard,abrasion-resistant, protective overcoat layer, typically containingcarbon, and a lubricant topcoat.

In operation of the magnetic recording medium, the polycrystallinemagnetic recording medium layer is locally magnetized by a writetransducer, or write head, to record and store information. The writetransducer creates a highly concentrated magnetic field which alternatesdirection based on the bits of information being stored. When the localmagnetic field produced by the write transducer is greater than thecoercivity of the recording medium layer, then the grains of thepolycrystalline recording medium at that location are magnetized. Thegrains retain their magnetization after the magnetic field produced bythe write transducer is removed. The direction of the magnetizationmatches the direction of the applied magnetic field. The magnetizationof the polycrystalline recording medium can subsequently produce anelectrical response in a read transducer, allowing the storedinformation to be read.

Thin film magnetic recording media are conventionally employed in diskform for use with disk drives for storing large amounts of data inmagnetizable form. Typically, one or more disks are rotated on a centralaxis in combination with data transducer heads. In operation, a typicalcontact start/stop (CSS) method commences when the head begins to slideagainst the surface of the disk as the disk begins to rotate. Uponreaching a predetermined high rotational speed, the head floats in airat a predetermined distance from the surface of the disk due to dynamicpressure effects caused by air flow generated between the slidingsurface of the head and the disk. During reading and recordingoperations, the transducer head is maintained at a controlled distancefrom the recording surface, supported on a bearing of air as the diskrotates. such that the head can be freely moved in both thecircumferential and radial directions, allowing data to be recorded onand retrieved from the disk at a desired position. Upon terminatingoperation of the disk drive, the rotational speed of the disk decreasesand the head again begins to slide agains the surface of the disk andeventually stops in contact with and pressing against the disk. Thus,the transducer head contacts the recording surface whenever the disk isstationary, accelerated from the static position, and duringdeceleration just prior to completely stopping. Each time the head anddisk assembly is driven, the sliding surface of the head repeats thecyclic sequence consisting of stopping, sliding against the surface ofthe disk, floating in the air, sliding against the surface of the disk,and stopping.

As a consequence of the above-described cyclic CSS-type operation, thesurface of the disk or medium surface wears off due to the slidingcontact if it has insufficient abrasion resistance or lubricationquality, resulting in breakage or damage if the medium surface wears offto a great extent, whereby operation of the disk drive for performingreading and reproducing operations becomes impossible. The protectiveovercoat layer is formed on the surface of the polycrystalline magneticrecording medium layer so as to protect the latter from friction andlike effects due to the above-described sliding action of the magnetichead. Abrasion-resistant, carbon (C)-containing protective coatings havebeen utilized for this purpose, and are typically formed by sputteringof a carbon target in an argon (Ar) atmosphere. Such amorphous carbon(a-C)-containing protective overcoat layers formed by sputtering haverelatively strong graphitic-type bonding, and therefore exhibit a lowcoefficient of friction in atmospheres containing water (H₂O) vapor,which characteristic is peculiar to graphite. However, the a-C layersproduced in such manner have very low hardness as compared with manyceramic materials such as are employed as slider materials of thin filmheads, and thus are likely to suffer from wear due to contact therewith.

In recent years, therefore, carbon-based protective overcoat layershaving diamond-like hardness properties (i.e., HV of about 1,000–5,000kg/mm²) have been developed, and films of diamond-like carbon (DLC)having a high percentage of diamond-type C—C bonding have been utilized.Such DLC films exhibit a high degree of hardness due to theirdiamond-like sp³ bonding structure, and in addition, exhibit theexcellent sliding properties characteristic of carbon, thus affordingimproved sliding resistance against sliders composed of high hardnessmaterials. Such DLC films are generally obtained by DC or RF magnetronsputtering of a carbon target in a gas atmosphere comprising a mixtureof Ar gas and a hydrocarbon gas, e.g., methane, or hydrogen gas. Thethus-obtained films exhibit DLC properties when a fixed amount ofhydrogen is incorporated therein. Incorporation of excessive amounts ofhydrogen in the films leads to gradual softening, and thus the hydrogencontent of the films must be carefully regulated.

Amorphous, hydrogenated carbon films (referred to herein as a-C:H films)obtained by sputtering of carbon targets in an Ar+H₂ gas mixtureexhibiting diamond-like properties have also been developed forimproving the tribological performance of disk drives; however, theelectrical insulating properties of such type films lead to undesirableelectrical charge build-up or accumulation during hard disk operationwhich can result in contamination, glide noise, etc. In order to solvethis problem without sacrifice or diminution of the advantageousmechanical properties of such a-C:H films, attempts have been made todope or otherwise incorporate nitrogen (N) atoms into the a-C:H films,in view of a substantial decrease in electrical resistivity and opticalband gap (E_(BG)) exhibited by such nitrogen-doped a-C:H films relativeto undoped films.

However, the continuous increase in areal recording density of magneticrecording media requires a commensurately lower flying height.Therefore, it would be advantageous to reduce the thickness of thecarbon-based protective overcoat layer to below about 100 Å withoutadverse consequences. Conventional sputtered a-C:H materials aredifficult to uniformly deposit and generally do not functionsatisfactorily at a thickness of about 100 Å or less. The use ofalternative deposition techniques for developing thinner and hardera-C:H layers having the requisite mechanical and tribological propertieshas been studied, such as chemical vapor deposition (CVD), ion beamdeposition (IBD), and cathodic arc deposition (CAD) techniques. Forexample, the IBD method can be utilized for forming hydrogenatedion-beam carbon films (referred to herein as i-C:H films) that exhibitsuperior tribological performance at thicknesses below about 100 Å.However, such films are insulating and, thus, suffer from theabove-described drawback of electrical charge build-up during hard diskoperation associated with sputtered a-C:H films.

Accordingly, there exists a need for an improved hard,abrasion-resistant material particularly suitable for use as aprotective overcoat layer in high-density magnetic recording media, anda method for manufacturing same, which method is simple, cost-effectiveand fully compatible with the productivity and throughput requirementsof automated manufacturing technology.

The present invention fully addresses and solves the above-describedproblems attendant upon the manufacture of ultra-thin,abrasion-resistant protective overcoat layers suitable for use with highdensity magnetic recording media, such as are employed in hard driveapplications, while maintaining full compatibility with all mechanicaland electrical aspects of conventional disk drive technology.

DISCLOSURE OF THE INVENTION

An advantage of the present invention is an improved hard,abrasion-resistant, ion beam-deposited (IBD) material.

Another advantage of the present invention is an improved protectiveovercoat material for magnetic recording media and comprising IBD,nitrogen-doped, hydrogenated carbon (i-C:HN).

Yet another advantage of the present invention is an improved magneticrecording medium including a protective overcoat layer comprised of IBDi-C:HN.

Still another advantage of the present invention is an improved IBDmethod for forming i-C:HN films or layers suitable for use as protectiveovercoat materials in magnetic recording media applications.

A still further advantage of the present invention is an improved methodfor regulating or controlling the amount of N dopant atoms contained inIBD i-C:HN films or layers.

Additional advantages and other features of the present invention willbe set forth in the description which follows and in part will becomeapparent to those having ordinary skill in the art upon examination ofthe following or may be learned from the practice of the presentinvention. The advantages of the present invention may be realized andobtained as particularly pointed out in the appended claims.

According to one aspect of the present invention, the foregoing andother advantages are obtained in part by providing a hard,abrasion-resistant material useful as a protective overcoat for amagnetic recording medium, which material comprises ion beam-deposited(IBD), nitrogen-doped, hydrogenated carbon (i-C:HN).

According to embodiments of the present invention, the mechanical andtribological properties of the IBD, i-C:HN material are substantiallythe same as IBD, undoped, hydrogenated carbon (i-C:H) but the electricalresistivity and optical band gap (E_(BG)) thereof are substantiallylower; the hardness is about 15–20 GPa, the density is about 1.5–2.5g/cm³, and the E_(BG) is less than about 1.8 eV.

In further embodiments according to the present invention, the materialcomprises about 5–25 at. % each of H and N and the E_(BG) is about1.4–1.6 eV; and a magnetic recording medium comprises a protectiveovercoat layer of the IBD i-C:HN material having a thickness less thanabout 100 Å.

Another aspect of the present invention is a method of forming a layerof an abrasion-resistant coating material, which method comprises:

(a) supplying an ion beam generator with a hydrocarbon gas of formulaC_(x)H_(y), where x=1–4 and y=2–10, and nitrogen (N₂) gas to generate anion beam comprising C, H, and N-containing ions; and

(b) directing the ion beam onto the surface of a substrate to deposit alayer of IBD i-C:HN thereon.

In embodiments according to the present invention, the mechanical andtribological properties of the IBD i-C:HN layer are substantially thesame as IBD, undoped i-C:H, but the electrical resistivity and E_(BG)are substantially lower; the hardness of the layer of IBD i-C:HN isabout 15–20 GPa, the density is about 1.5–2.5 g/cm³, the electricalresistivity is less than about 10¹¹ ohm-cm, and the E_(BG) is less thanabout 1.8 eV.

According to further embodiments of the present invention, the layer ofIBD i-C:HN comprises about 5–25 at. % each of H and N and the E_(BG)thereof is about 1.4–1.6 eV; step (a) comprises supplying the ion beamgenerator with acetylene (C₂H₂) and N₂ gases at a C₂H₂:N₂ flow ratio offrom about 1:1 to about 20:1; step (b) comprises depositing a layer ofthe IBD i-C:HN less than about 100 Å thick on the substrate surface; andthe substrate comprises a magnetic recording medium.

According to still further embodiments of the present invention, step(a) further comprises supplying an end-Hall-type ion beam generator withan inert gas and generating the ion beam under the following conditions:

anode current: 4–12 amperes magnet current: 2–8 amperes anode voltage:60–100 volts total gas flow: 100–200 sccm hydrocarbon gas flow: 20–40sccm nitrogen gas flow: 2–30 sccm pressure: 1.5–4.5 mTorr

According to still another aspect of the present invention, a method offorming a layer of an abrasion-resistant protective coating materialcomprising IBD i-C:HN is provided, which method comprises the steps of:

(a) supplying a hydrocarbon gas of formula C_(x)H_(y), where x=1–4 andy=2–10, and N₂ gas to an ion beam generator to generate an ion beamcomprising C, H, and N-containing ions;

(b) providing a substrate having a surface for deposition of said layerof IBD i-C:HN thereon;

(c) directing the ion beam onto the surface of the substrate to depositthe layer of IBD i-C:HN thereon; and

(d) controlling or regulating the content of N atoms in the IBD i-C:HNlayer by controlling the hydrocarbon gas:N₂ gas flow ratio supplied tothe ion beam generator.

In embodiments according to the present invention, step (a) comprisessupplying acetylene (C₂H₂) gas to the ion beam generator as thehydrocarbon gas; and step (d) comprises controlling the C₂H₂:N₂ gas flowratio to be within from about 1:1 to about 20:1; whereby step (c)comprises depositing a layer of IBD i-C:HN comprising about 5–25 at. %each of H and N; step (c) further comprises depositing a layer of IBDi-C:HN less than about 100 Å thick and having a hardness of about 15–20GPa, density of about 1.5–2.5 g/cm³, electrical resistivity less thanabout 10¹¹ ohm-cm, and E_(BG) of about 1.4–1.6 eV; and step (c)comprises providing a magnetic recording medium as the substrate.

According to further embodiments of the present invention, step (a)further comprises utilizing an end-Hall-type ion beam generator,supplying the ion beam generator with an inert gas, and generating theion beam under the following operating conditions:

anode current: 4–12 amperes magnet current: 2–8 amperes anode voltage:60–100 volts total gas flow: 100–200 sccm hydrocarbon gas flow: 20–40sccm nitrogen gas flow: 2–30 sccm pressure: 1.5–4.5 mTorr

Additional advantages of the present invention will become readilyapparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present invention are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present invention. As will be described, the presentinvention is capable of other and different embodiments, and its severaldetails are susceptible of modification in various obvious respects, allwithout departing from the spirit of the present invention. Accordingly,the description is to be regarded as illustrative in nature, and not aslimitative.

DESCRIPTION OF THE INVENTION

The present invention addresses and solves problems arising from theelectrical insulating properties of hard, abrasion-resistant a-C:H filmsutilized as protective overcoat layers in magnetic recording media,e.g., hard disks. More specifically, the inventive methodology providesnovel materials: (1) having the advantageous mechanical and tribologicalproperties associated with a-C:H; (2) usable at the thinner thicknessesprovided by ion beam deposition (IBD) techniques; and (3) havingsufficiently reduced insulative properties vis-à-vis conventionalsputtered a-C:H and IBD i-C:H materials as to eliminate, or at leastminimize, deleterious electrical charge build-up during hard diskoperation.

According to the present invention, the problems and difficultiesattendant upon the use of a-C:H and IBD i-C:H materials as protectiveovercoat layers in the manufacture of magnetic recording media, such ashard disks, are eliminated, or at least minimized, by nitrogen (N)doping of IBD i-C:H in an amount sufficient to substantially reduce theelectrical resistivity from that of undoped IBD i-C:H, while maintainingthe advantageous mechanical and tribological properties thereof as wellas the ability to employ films of reduced thickness. e.g., less thanabout 100 Å, such as are required for use with ultra high-densitymagnetic recording media.

The IBD method has been successfully employed to produce hydrogenatedion-beam carbon (IBD i-C:H) films that exhibit superior tribologicalperformance as compared to conventional sputtered a-C:H films, atthicknesses below about 100 Å. However, inasmuch as these IBD i-C:Hfilms are electrically insulative, they are subject to the same problemof charge build-up associated with the conventional sputtered a-C:Hfilms.

The present invention is based upon the decrease in resistivity andoptical band gap (E_(BG)) observed when sputtered a-C:H films are dopedwith nitrogen (N) atoms to form a-C:HN films (J. H. Kaufman et al.,Phys. Rev. B39, 13053 (1989); O. Amir et al., J. Appl. Phys. 70, 4958(1991)). The resistivity of undoped, IBD i-C:H was measured and comparedwith that of sputtered, N-doped, a-C:HN films. whereby it was determinedthat the sputtered, N-doped a-C:HN films were substantially lessresistive than the IBD, undoped films. Therefore, in order to reduce theresistivity (and E_(BG)) of IBD i-C:H films in order to increase theirusefulness as ultra-thin (i.e., less than about 100 Å thick) protectiveovercoat layers for magnetic recording media such as hard disks,N-doped, IBD i-C:HN films were produced by supplying a mixture ofhydrocarbon (C_(x)H_(y), where x=1–4 and y=2–10) gas and nitrogen (N₂)gas to the ion beam generator. The nitrogen atom (N) content of theproduced films was readily controlled within from about 15 to about 25at. % by varying the ratio of the flow rate of the hydrocarbon gas tothat of the N₂ gas. Hydrogen atom (H) content of the obtained films wasless dependent upon variation in hydrocarbon gas/N₂ gas flow ratio, andgenerally was within from about 2 to about 25 at. %. The N-doped IBDi-C:HN films advantageously retained the superior mechanical andtribological properties of undoped, IBD i-C:H films at ultra-thinthicknesses, i.e., hardness of from about 15 to about 20 GPa, density ofabout 1.5–2.5 g/cm³, while exhibiting substantially reduced resistivityof less than about 10¹¹ ohm-cm and E_(BG) of less than about 1.8 eV,e.g., about 1.4–1.6 eV. Representative results of experiments conductedfor determining the dependence of film properties on variation of thehydrocarbon gas/N₂ gas flow ratio are summarized in Table 1, wherein thehydrocarbon gas C_(x)H_(y) is acetylene (C₂H₂).

TABLE 1 C_(x)H_(y) only C_(x)H_(y):N₂ = 3:1 C_(x)H_(y):N₂ = 1:1 H (at.%) 20 16.5 15.5 N (at %) 0 10.5 16 E_(BG) (eV) 1.79 1.52 1.46

As is evident from Table 1, the N atom content of IBD i-C:HN films isreadily regulated and/or controlled by appropriate selection of theC_(x)H_(y):N₂ flow ratio, i.e., the N atom content can be increased byincreasing the flow rate of N₂ gas supplied to the ion beam generator,relative to that of the hydrocarbon gas. In addition, the value ofE_(BG) is progressively decreased with increase in the flow rate of N₂gas, and hence with increasing N atom content of the films.

Any of various ion beam sources can be employed to achieve theobjectives of the present invention, including, inter alia, broad- andnarrow-beam sources, Kaufman-type DC discharge sources, RF or microwaveplasma discharge sources, electron-cyclotron resonance (ECR) sources,gridless sources such as the end-Hall source of U.S. Pat. No. 4,862,032,the disclosure of which is incorporated herein by reference, and hollowcathode ion sources. By way of illustration, but not limitation, IBDi-C:HN films and coatings containing from about 5 to about 25 at. % Natoms and having mechanical, tribological, electrical, and opticalproperties suitable for extended use as ultra-thin, i.e., less thanabout 100 Å thick, e.g., a thickness of about 20 to about 80 Å,protective overcoat layers on substrates comprising magnetic recordingmedia were obtained by use of an end-Hall ion beam source (Diamonex,Inc., Allentown, Pa.) operated under the following conditions:

anode current: 4–12 amperes magnet current: 2–8 amperes anode voltage:60–100 volts total gas flow: 100–200 sccm hydrocarbon gas (C₂H₂) flow:10–30 sccm pressure: 1.5–4.5 mTorr

The present invention provides a number of advantages over theconventional sputter-deposited a-C:H and a-C:HN materials for use asabrasion-resistant protective overcoat layers for magnetic recordingmedia, such as hard disks. More specifically, the IBD i-C:HN filmsformed according to the inventive methodology are substantially lesssusceptible to deleterious charge-build up effects associated with themore resistive a-C:H materials and can be employed at ultra-thinthicknesses (i.e., less than about 100 Å) at which neither the a-C:H orthe a-C:HN materials are usable, thereby meeting the requirement of verythin overcoat layers for high density magnetic recording media. Inaddition, the inventive methodology is fully compatible with all otheraspects of magnetic recording media technology and is readily adaptedfor use in automated manufacturing processes.

In the previous description, numerous specific details are set forth,such as specific materials, structures, reactants, processes, etc., inorder to provide a better understanding of the present invention.However, the present invention can be practiced without resorting to thedetails specifically set forth. In other instances, well knownprocessing materials, structures, and techniques have not been describedin detail in order not to unnecessarily obscure the present invention.

Only the preferred embodiments of the present invention and but a fewexamples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is susceptibleof changes and/or modifications within the scope of the inventiveconcept as expressed herein.

1. An abrasion-resistant material useful as a protective overcoat layerfor a magnetic recording medium, which material comprises ionbeam-deposited, nitrogen-doped, hydrogenated carbon (IBD i-C:HN),wherein the hardness of the IBD i-C:HN layer being about 15–20 GPa, thedensity being about 1.5–2.5 g/cm³, the electrical resistivity being lessthan about 10¹¹ ohm-cm, and the E_(BG) being about 1.4–1.6 eV.
 2. Thematerial according to claim 1, comprising about 5–25 at. % each of H andN.